Why I get a System.NullReferenceException on this code?

Question

I have this code :

IList<MyObject> myObjects = new List<MyObject>();
if (param != null)
{
    myObjects = (from ... LINQ1 ...).ToList();
}
else
{
    myObjects = (from ... LINQ2 ...).ToList();
}

foreach (MyObject myObject in myObjects)
{
}

when the foreach start, I get a System.NullReferenceException. Why? And how can I fix it? Looks strange...

You haven't given enough code for us to know. Perhaps one of your queries contains a null reference which you're using in a comparison etc? A short but complete program which demonstrates the problem would help. Also, there's no point in giving `myObjects` an initial value if you're then just going to overwrite it. — Jon Skeet, Feb 06 '12 at 11:55
Please post the entire code. The problem is probably in the code snippets that you have deleted. — Anders Marzi Tornblad, Feb 06 '12 at 11:56
Give us the stack trace of the exception, so we can investigate the problem — Mohammad Dehghan, Feb 06 '12 at 11:56
may be you have to use List instead of IList... Refer to this http://stackoverflow.com/questions/1367539/ilistint-vs-listint — Sai Kalyan Kumar Akshinthala, Feb 06 '12 at 12:00
Impossible fto help. The error is somewhere ni: "(from ... LINQ2 ...)" .- the evaluation of that must throw a null pointer exception internally. — TomTom, Feb 06 '12 at 12:11

score 1 · Accepted Answer · answered Feb 06 '12 at 11:58

1

Your object is being overwritten by one of the LINQ queries which is most likely returning a null value.

Also is myObjects meant to be an IList<MyObject> or List<MyObject> ?

answered Feb 06 '12 at 11:58

ChrisBD

9,104
3
22
35

How? "...).ToList();" is supposed to return a list, even if empty. – TomTom Feb 06 '12 at 11:59
@TomTom, yes but it is possible to write a linq query that returns a null ref exception, because of something in the query, the To List is just enumerating through the items and executing the expression – Ben Robinson Feb 06 '12 at 12:03
Yes, that is possible- that the not shown part throws a null reference WHEN EVALUATING. – TomTom Feb 06 '12 at 12:10
@TomTom, are you saying that .ToList() on a null value is supposed to return an empty list? – Davin Tryon Feb 06 '12 at 12:10
1

No, i said that ToList always returns a list, even when no element is in teh query. So, the result will never be a null. BUT: as Ben said, it is possible something in the query throws the exception when evaluating. – TomTom Feb 06 '12 at 12:11

Why I get a System.NullReferenceException on this code?

1 Answers1